The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment

Journal name:
Nature
Volume:
477,
Pages:
195–198
Date published:
DOI:
doi:10.1038/nature10399
Received
Accepted
Published online

Many precious, ‘iron-loving’ metals, such as gold, are surprisingly abundant in the accessible parts of the Earth, given the efficiency with which core formation should have removed them to the planet’s deep interior1. One explanation of their over-abundance is a ‘late veneer’—a flux of meteorites added to the Earth after core formation as a ‘terminal’ bombardment that culminated in the cratering of the Moon2. Some 3.8 billion-year-old rocks from Isua, Greenland, are derived from sources that retain an isotopic memory of events pre-dating this cataclysmic meteorite shower3, 4. These Isua samples thus provide a window on the composition of the Earth before such a late veneer and allow a direct test of its importance in modifying the composition of the planet. Using high-precision (less than 6 parts per million, 2 standard deviations) tungsten isotope analyses of these rocks, here we show that they have a isotopic tungsten ratio 182W/184W that is significantly higher (about 13 parts per million) than modern terrestrial samples. This finding is in good agreement with the expected influence of a late veneer. We also show that alternative interpretations, such as partial remixing of a deep-mantle reservoir formed in the Hadean eon5, 6 (more than four billion years ago) or core–mantle interaction7, do not explain the W isotope data well. The decrease in mantle 182W/184W occurs during the Archean eon (about four to three billion years ago), potentially on the same timescale as a notable decrease in 142Nd/144Nd (refs 3 and 6). We speculate that both observations can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection.

At a glance

Figures

  1. [epsi]182W measurements of Isua and post-Archean samples.
    Figure 1: ε182W measurements of Isua and post-Archean samples.

    The upper panel shows results for Isua and post-Archean samples analysed with high-precision, quintuple measurements. Individual quintuple measurements are plotted as grey circles and averaged, repeated quintuple measurements are plotted as black circles (with 2s.e. uncertainties). The lower panel shows single measurements of recent oceanic basalts thought to be derived from deep mantle plumes. The average for each sample grouping is shown as a solid vertical line (with dashed lines indicating ±2s.d.). We note the significant deviation of the ε182W Isua samples from zero. Also shown are average values (squares) for Isua and post-Archean samples corrected for minor non-zero ε183W in some samples (see text and Supplementary Information).

  2. Model estimates of [epsi]182W in modern mantle.
    Figure 2: Model estimates of ε182W in modern mantle.

    a, The effect of adding variable mass fractions (relative to the total silicate Earth) of late veneer to a Hadean mantle with ε182W = 0.13, as determined in this study. In this calculation we use a current bulk silicate earth value of tungsten concentration from ref. 28 and compositions of LL29 and H30 ordinary chondrites, taken to span the range of probable impacting material11. The fraction of chondritic material added to the mantle in the late veneer has independently been estimated5 from HSE abundances to be 0.005, in keeping with the range of possible solutions shown here (0.002 to 0.009). b, Modelled minimum ε182W for the modern mantle calculated from 5,000,000 Monte Carlo simulations of a simple model (see Supplementary Information for full description) that satisfies the 142Nd,143Nd isotope constraints of Hadean and modern mantle by partial remixing of a portion of an EER (see text). Results are shown using two sets of partition coefficients D, appropriate for forming a hidden reservoir during deep and shallow mantle fractionation15, respectively. The values of modern ε182W require an assumption about ε182W at the time of hidden reservoir formation. We use a chondritic reference as a conservative minimum estimate and yet still find no model ε182W as low as the modern values of zero. Thus the partial remixing model yields no plausible solutions.

References

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Author information

Affiliations

  1. Bristol Isotope Group, School of Earth Sciences, Wills Memorial Building, Queens Road, University of Bristol, Bristol BS8 1RJ, UK

    • Matthias Willbold &
    • Tim Elliott
  2. Department of Earth Sciences, South Parks Road, University of Oxford, Oxford OX1 3AN, UK

    • Stephen Moorbath

Contributions

Samples from Isua were collected by S.M. Analytical development and sample analyses were carried out by M.W. Modelling and manuscript preparation was carried out by T.E. and M.W. All authors contributed to discussing the results and implications.

Competing financial interests

The authors declare no competing financial interests.

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Supplementary information

PDF files

  1. Supplementary Information (1.2M)

    This file contains Supplementary Text and Data, Supplementary Figures 1-4 with legends and Supplementary Tables 1-5 (see separate excel files for Supplementary Tables 6 and 7).

Excel files

  1. Supplementary Table 6 (1M)

    This table shows the results of Monte-Carlo simulations using shallow mantle partition coefficients for hidden reservoir formation.

  2. Supplementary Table 7 (1.2M)

    This table shows the results of Monte-Carlo simulations using deep mantle partition coefficients for hidden reservoir formation.

Additional data